The immunoglobulin Dob structure has been determined at the quaternary level. The techniques developed in solving the Dob structure have been applied to obtain a solution of the Fab of J539, a galactan binding mouse myeloma protein at 4.5 A resolution. The McPC 603 Fab model has been completely rebuilt on the computer graphics system at the UNC, Chapel Hill, and is now being prepared for refinement at 3.1A resolution. Photographic data to 2.7A have been collected and will be used when processed.